The present invention relates to a light source apparatus with an electrodeless discharge tube, and more particularly to one suitable for atomic absorption analysis apparatus, atomic fluorescence analysis apparatus, etc.
A spectroscopic electrodeless discharge tube is normally a lamp made of quartz being filled with rare gas and luminous metal, salt with metal or amalgam. The electrodeless discharge tube, as the hollow cathode lamp, can be used as a light source for atomic absorption analysis apparatus, because it can be used as a bright-line spectrum source. Few electrodeless discharge tubes, however, have been employed as the light source for the atomic absorption analysis apparatus. The main reason for this is that the conventional electrodeless discharge tube has a poor absorbance compared with the hollow cathode lamp and that it fails to have a light intensity stable for a long time. The poor absorbance is probably due to self-absorption and a broad doppler width.
Elementary vapour pressure in the electrodeless discharge tube must be kept about 10.sup.-.sup.3 Torr for lighting the electrodeless discharge tube. Under such vapour pressure, the elementary vapour in the discharge tube acts to constantly condense on the low temperature portion of the discharge tube inside wall, under the vapour pressure of phase equilibrium. Thus, if the low-temperature portion is localized near the lighting portion, a portion from which the light emitted by the elements is extracted from the discharge tube, the elementary vapour condenses on the discharge tube inside wall corresponding to the lighting portion, thereby resulting in reduction of light intensity. If the low temperature portion is localized on the inside wall of the discharge tube portion wound by the high-frequency coil, the metal vapour deposits thereon and serves to shield the high-frequency energy applied by the coil. The result is that the electrodeless discharge tube stops to luminescing.
The intensity of the light emitted from the elements in the electrodeless discharge tube depends on the vapour pressure of the elements filled therein, and the vapour pressure is based on the surface temperature of the elementary drops existing in the discharge tube. An action of the elementary drops when the discharge tube is lit up will be explained referring to the case of a mercury electrodelss discharge tube. Spattered mercury drops were inherent to such conventional tube. Upon lighting of the discharge tube, the vapour from the mercury drops laid on the highest-temperature portion of the discharge tube diffuses into the space of the tube and condenses on the inside wall of the tube at a low temperature. The diffusion of the mercury drops laying on that portion continues until the mercury drops disappear. Following this, the mercury drops laying on the discharge tube inside wall where the temperature is high next to that of the former portion, are vapoured to diffuse in the electrodeless discharge tube and then to condense on the low-temperature temperature portion of the discharge tube wall. This process will be repeated and, finally, the mercury drops remain at only the low-temperature portion of the tube wall. For this reason, the high intensity of the discharge tube remains unstable for a short time after the lighting-up of the tube. The spattered low-temperature portions along the discharge tube wall incur the reduction of the light intensity and more adversely the stopage of the luminescence in the tube.
The electrodeless discharge lamp may also be used as a light source for atomic absorption analysis apparatus applying the Zeeman effect. A detailed Zeeman atomic absorption analysis is set forth in the U.S. Patent Application No. 474,812, filed in 1972. In the Zeeman atomic absorption analysis, one of the components of a spectral line split by a magnetic field applied is used for light of a sample, while another one is used for light of a reference. The electrodeless discharge lamp used as a light source for the known Zeeman atomic absorption analysis apparatus physically had a diameter of 10 mm or more, and a tube with a high-frequency coil therearound is provided in the gap between the magnetic poles of the magnet. The gap permits the light emitted to travel outside. Such constructed light source involves many problems, other than above mentioned ones: The magnet used must be of large size, due to the broad gap between the magnetic poles; The guidance of the light emitted from the elements in the discharge tube to the exterior is continued in the direction of the magnetic field applied to the elements; A uniform magnetic field can not be obtained because of the opening bored in the magnet.